The invention relates generally to processes for cleaning lime having high levels of contaminants. More particularly, the invention relates to a process for cleaning lime containing manganese oxide contaminants so that such cleaned lime can be used in the Bayer Process to lower the iron content of Bayer liquor without causing manganese oxide contamination in the product alumina.
Alumina is conventionally recovered from bauxite and similar alumina-bearing ores by a process generally referred to as the Bayer Process. The process recovers alumina from the ore by digesting the ore with caustic liquor which dissolves most of the alumina into the liquor. That which remains undissolved is generally referred to as red mud. Red mud primarily contains unwanted ore constituents, such as iron, titanium and desilication product. However, some undissolved alumina is also usually present. After digestion, the liquor is typically removed from the red mud by decantation and filtration. Aluminum trihydroxide is then precipitated from the remaining liquor and calcined to form alumina. Precipitation usually involves cooling and mixing of the liquor (which is supersaturated and sometimes called green or pregnant liquor) with a slurry of aluminum trihydroxide or alumina trihydrate in spent Bayer liquor which acts as a seed to induce formation of its own species. The slurry is then typically pumped through a classification system which separates the slurry into a coarse fraction of crystallized aluminum trihydroxide and a fine fraction of remaining aluminate liquor. The liquor, at this point, is typically referred to as spent Bayer liquor. The spent Bayer liquor is then generally concentrated by evaporation and recycled to the digester where it is mixed with incoming ore.
Certain types of bauxite contain higher than normal levels of pyrite, siderite and silica. During digestion of such bauxite, the silica contained therein reacts with aluminate and caustic species in the digester to form an insoluble zeolite type compound generally referred to as the desilication product. The desilication product results in a loss of soda and alumina by chemically binding the soda and alumina. To recover such lost alumina and soda, the desilication product is subjected to a lime soda sintering process such as that disclosed in U.S. Pat. No. 3,796,789 to Adams which is hereby incorporated by reference. Such sintering processes often use oil in the sintering kilns for economic reasons instead of natural gas. Oil, however, causes additional sulfur compounds to form in the spent liquor. When such spent liquor is recycled to the digester, as in the conventional Bayer Process, these additional sulfur compounds, together with that contributed by the pyrite from bauxite, apparently peptize the Fe(II) particles of the siderite to form charged Fe.sub.2 O.sub.3 colloidal species of highly hydrated states which are too small to be removed completely during the filtration process and, thus, are precipitated along with the aluminum trihydroxide. Alumina having high levels of Fe.sub.2 O.sub.3 is characterized by having an undesirable pink color instead of the typical commercial grade white. When this Fe.sub.2 O.sub.3 level exceeds 0.05% by weight in the calcined product, i.e., alumina, the quality of chemical products produced therewith is adversely affected.
Numerous methods have been reported in the literature for lowering the iron level in Bayer liquor. These include, for example, filtration through the use of iron grit or sand, step precipitation, filter precoat, magnetic separation, oxidation of the sulfur compound in the liquor, addition of zinc to form slightly soluble zinc compounds, and the addition of lime to the green liquor, such as that disclosed by Adams in U.S. Pat. No. 3,796,789. All of these methods, except the last one, are generally unattractive from an economic standpoint in that they involve additional equipment and a more complicated treatment process such as regeneration of the additive which may also, depending on the additive, introduce additional contaminants in the liquor stream. In addition, while most of these methods have been demonstrated to lower Fe.sub.2 O.sub.3 content in the liquor, most require spot checking to see whether they are also effectively lowering Fe.sub.2 O.sub.3 level in the precipitated alumina without introducing other contaminants. For example, the addition of commercial lime to the settler overflow liquor seems to be harmless, yet if it is not carefully watched and controlled, it will introduce excessive contaminants in the precipitated aluminum trihydroxide, thereby rendering the method commercially unattractive.
Lime's ability to lower the iron content of the liquor is believed to be due to its presence in sufficient concentration which causes rapid precipitation of its hydroxide, carbonate and aluminate, which enmesh the colloidal iron species as they are formed. This behavior is generally referred to as the "enmeshment" mechanism.
While the addition of lime is an effective way of lowering iron content in Bayer liquor, many desirable (i.e., inexpensive and locally available) sources of lime contain high levels of contaminants, such as manganese oxide, which are not trapped by the enmeshment mechanism. Manganese oxide is particularly troublesome because, in addition to avoiding "enmeshment", it precipitates out with the aluminum trihydroxide. If present in contaminating quantities, it will discolor the product alumina, giving it a pink, instead of white, look. Whiteness generally indicates that the alumina is relatively pure. If, however, white alumina could be produced with such contaminated lime, production costs could be lowered. Accordingly, an economical method of producing white, commercial grade alumina using such contaminated lime would be most desirable.